The system in accordance with this invention is preferentially operated in the “brake by wire” operating mode, and can be operated in at least one fall-back operating mode in which only operation by the vehicle driver is possible, having
a) a brake pedal for actuating a pedal decoupling unit having a housing, having two pistons which are arranged in series and which, in the housing, delimit two pressure chambers, which pistons are subjected to an actuating force (pedal force) when the brake system is actuated by the driver, and which pistons are positioned in an initial position by restoring springs when the brake pedal is not actuated,
b) a pressure medium reservoir which is assigned to the pressure chambers and which is at atmospheric pressure,
c) a travel measuring device which measures the actuating travel of the brake pedal or of a piston which is connected to the brake pedal,
d) a travel simulator with a simulator enable valve which, in the “brake by wire” operating mode, imparts the familiar brake pedal feel to the vehicle driver, the travel simulator being hydraulically connected to one of the pressure chambers, and the action of which travel simulator can be deactivated in the fall-back operating mode,
e) an electrohydraulic pressure generating device which outputs a brake system pressure,
f) a pressure modulation unit which has one inlet valve and one outlet valve per wheel brake for setting wheel-specific brake pressures derived from the brake system pressure, wherein the inlet and outlet valves output or transmit the brake system pressure when in the non-actuated state, having cut-off valves for cutting off the pressure chambers from the pressure modulation unit, and
g) an electronic control and regulating unit.
A brake system of a prior art type is known from EP 1 950 112 A1. It is considered to be a severe disadvantage of that known brake system that, in the event of a failure of the electric drive of the electrohydraulic pressure generating device during a braking process, a restoring process of the piston thereof results in a considerable lengthening of the brake pedal travel, which is perceived by the vehicle driver as a brake failure.
It is therefore an object of the present invention to improve a brake system of the type specified above such that a failure of the on-board electrical system of the motor vehicle equipped with the brake system does not lead to confusion for the vehicle driver.
The object is achieved according to the invention by means of the technical features specified herein.